Current infrared sensors, including imaging arrays for night vision applications, generally employ a platform coated with a material sensitive to infrared (IR) radiation. As shown in FIG. 1, during the operation of a conventional IR sensing apparatus 600, as IR energy 601 strikes an IR support platform 602, the IR sensitive coating 603 disposed on the platform 602 undergoes an increase in temperature, which causes a change in some physical parameter or electrical characteristic, generally electrical resistance. An electronic circuitry 604, located close to the platform 602 and which may be integrated with a substrate 605, detects this change in resistance or any other IR sensitive parameter. i.e., electrical or mechanical characteristic. The electrical circuit 604 is in electrical contact with the IR sensitive coating 603. There is also a mechanical connection or bridge 606 between the substrate 605 and the platform 602. Both the electrical contact and mechanical bridge provide a path for heat, which then contributes to the IR energy level received by the coating 603 to flow out of the platform 602, thereby lowering the sensitivity of the sensor. Heretofore, a great deal of effort has gone into making the thermal resistance of the mechanical suspension and electrical contacting as high as possible so as to increase the sensitivity of the detector.
In particular, there has been outstanding work in recent years in the area of uncooled bolometers. Several groups have reported bolometer arrays with noise equivalent temperature differences (NETD) less than 100 mK (See references: W. J. Parrish, J. T. Woolaway, G. Kincaid, J. L. Heath, and J. D. Frank, Low Cost 160×128 Uncooled Infrared Sensor Array, SPIB Conference on Infrared Electronics IV, Orlando, Fla. April 1998, pp. 111-119: P. Kruse, R. Dodson, S. Anderson, L. Kantor, M. Knipfer, and T. McManus, Infrared Imager Bmploying 160×120 Pixel Uncooled Bolometer Array, SPIE Conference on Infrared Technology and Applications XXIV, San Diego, Calif., pp. 572-577; R. A. Wood, Uncooled Thermal Imaging with Monolithic Silicon Focal Planes, Proc. SPIB, vol. 2020, 1993, pp. 322-329; and P. W. Kruse, The Design of Uncooled Infrared Imaging Arrays, Proc. SPIB, vol. 2746, 1996, pp. 34-37 (the foregoing publications are hereby incorporated by reference herein)). The August 1999 DARPA/MTO Optoelectronics Review anticipates that 640×480 arrays with 50 mK NETD will be developed in the year 2000. All of the systems described in published works to date still have undesirably large sensitivity limitations due to thermal conduction losses.
As discussed in, U. Ringh, C. Jansson, and K. Liddiard, Readout Concept Employing a Novel On Chip 16 bit ADC for Smart IR Focal Plane Arrays, Proc. SPIE, vol. 2745, 1996, pp. 99-110 (the foregoing publication is hereby incorporated by reference herein), bolometer sensitivity is fundamentally limited by the detector temperature fluctuation noise:P2ΔT=4kGT2where P2ΔT is the noise power, T is temperature, and G is the thermal conductance of the detector to thermal “ground.” NETD has also been shown to be proportional to G1/2 in Kruse. Reducing thermal conductivity is therefore of great importance to the overall bolometer sensitivity. Wood has reported a thermal conductivity of 80 n W/°C. and an NETD of 40 mK.
A wide range of commercial applications will emerge for IR sensors as they become less expensive. Night navigation systems for cars and ships, systems for IR detection through walls and smoke, night sight systems for law enforcement, and night time oil spill and pollution detection systems are but some of the many potential commercial applications.
There is therefore a need in the art for an IR sensing system having improved sensitivity whereby the thermal conduction losses are substantially or completely eliminated, but for limited radiation loss through the surrounding vacuum.
Examples of conventional IR sensors and related art are disclosed in the following list of U.S. Patents, and are herein incorporated by reference:
Hornbeck-5,021,663;Hornbeck-B1 5,021,663;Cole-5,286,976;Keenan-5,367,167;Gates-5,554,849;Koskinen-5,589,689;Gerard-5,602,393;Butler, et al.-5,821,598;Butler, et al.-5,850,098;Morris-5,900,799;Vilain, et al.-5,912,464;Wada, et al.-5,966,590;Parrish, et al.-6,028,309